1. The Field of the Invention
The present invention generally relates to x-ray tube devices. In particular, the present invention relates to an adjustable collimator that can be used to selectively control the size and shape of an x-ray signal pattern that is emitted from an x-ray tube, and at the same time minimize the amount of off-focal radiation that is emitted.
2. The Related Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis.
Regardless of the applications in which they are employed, x-ray devices operate in similar fashion. In general, x-rays are produced when electrons are emitted, accelerated, and then impinged upon a material of a particular composition. This process typically takes place within an evacuated enclosure of an x-ray tube. Disposed within the evacuated enclosure is a cathode, or electron source, and an anode oriented to receive electrons emitted by the cathode. The anode can be stationary within the tube, or can be in the form of a rotating annular disk that is mounted to a rotor shaft which, in turn, is rotatably supported by a bearing assembly. The evacuated enclosure is typically contained within an outer housing, which also serves as a coolant reservoir.
In operation, an electric current is supplied to a filament portion of the cathode, which causes a cloud of electrons to be emitted via a process known as thermionic emission. A high voltage potential is placed between the cathode and anode to cause the cloud of electrons to form a stream and accelerate toward a focal spot disposed on a target surface of the anode. Upon striking the target surface, some of the kinetic energy of the electrons is released in the form of electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials with high atomic numbers (xe2x80x9cZ numbersxe2x80x9d) are typically employed. The target surface of the anode is oriented so that the x-rays are emitted through windows defined in the evacuated enclosure and the outer housing. The emitted x-ray signal is then directed towards an x-ray subject, such as a medical patient, so as to produce an x-ray image.
Unfortunately, a portion of the x-rays produced within an x-ray tube do not originate at the focal spot of the anode. In fact, a significant portion of the x-rays are emitted from regions of the anode other than the focal spot; these x-rays are commonly referred to as xe2x80x9coff-focalxe2x80x9d radiation. This off-focal radiation can substantially degrade the image quality that is generated from the x-ray signal that is emitted from the x-ray tube, such as in a computed tomography device (CT scanner). Thus, there is a need to equip the x-ray tube generating device with a device that minimizes the amount of off-focal radiation that is emitted.
One device used to minimize off-focal radiation is an x-ray collimator. A typical x-ray beam collimator assembly comprises a mass of x-ray attenuating material, such as lead, with an opening defined therein. The collimator is then aligned with the window of the x-ray tube so that on-focus radiation (i e., x-rays that originate at the focal spot) can pass through the opening, and off-focal radiation is blocked by the x-ray attenuating portion of the collimator. Due to the manner in which x-rays are emitted from the anode, the collimator must be positioned as close as possible to the anode to block all off-focal radiation. However, existing collimator designs are positioned too far from the anode and the typically allow some off-focal radiation to escape.
In addition to minimizing the amount of off-focal radiation that is emitted, there is often a need to control the size and dimension of the pattern of the x-ray signal that exits the x-ray tube. The ideal dimensions of an x-ray signal are often dependent on the particular application involved. For example, in certain types of diagnostic radiology, x-ray signal having a relatively large pattern is used to produce images of relatively large portions of a patient""s body. At other times, a smaller and more focused x-ray beam signal is used to produce detailed images of relatively small portions of the patient""s body, such as regions of the head, for instance.
Thus, there is a need in the art for an x-ray tube collimation assembly that minimizes the amount of off-focal radiation that is emitted, so as to insure an accurate x-ray image. Moreover, it would be desirable if the collimation assembly was adjustable in a manner that permits selective control over the size, shape and dimensions of the x-ray signal pattern that is emitted.
The present invention has been developed in response to the above and other needs in the art. Briefly summarized, embodiments of the present invention are directed to a collimator assembly that minimizes the amount of off-focal radiation that is emitted from the x-ray generating device. Moreover, of the collimator assembly is capable of selectively altering the dimensions of an x-ray signal pattern that is emitted.
In one preferred configuration, the collimator assembly includes a primary x-ray passage area that is configured to allow an x-ray signal to pass and that is collimated to have a first predetermined shape and dimension. In a preferred embodiment, this primary x-ray passage area is shaped so as to provide an x-ray signal having pattern that is relatively broad, such as could be used to create a diagnostic x-ray image of relatively large portions of a patient""s body, such as whole body CT scans. In addition, in a preferred embodiment the collimator assembly is also capable of selectively defining a secondary x-ray passage area. This secondary passage area is configured to allow an x-ray signal to pass having a pattern with a different shape and dimension. In a preferred embodiment, the secondary x-ray passage area is configured to emit a smaller, more focused x-ray beam. This smaller beam is useful in creating more detailed images of small portions of a patient""s body, such as a head scan in a CT scanner. Importantly, in either configuration, the collimator assembly is configured in a manner so as to minimize the emission of any off-focal radiation from the x-ray generating device, thereby insuring a high quality x-ray image.
In one presently preferred embodiment, the adjustable x-ray beam collimator assembly comprises a collimator plate having a slot that defines the primary x-ray passage area. In one embodiment, the slot has a rectangular shape, but other geometric shapes could also be utilized, depending on the needs of any particular application. The collimator plate is preferably at least partially comprised of an x-ray attenuating material that is capable of blocking off-focal radiation. In a preferred embodiment, the collimator plate is mounted at an end of a base member. This base member is mounted to the outer housing of the x-ray tube in a manner so that the plate is positioned immediately adjacent to the rotating anode. This ensures that a majority of the off-focal radiation is blocked. Of course, the collimator assembly could be mounted differentlyxe2x80x94including at points external to the outer housing of the x-ray tubexe2x80x94depending on the needs of the particular application.
The collimator assembly further includes a blocking member that can be selectively positioned with respect to the first x-ray passage area so as to form the secondary x-ray passage area. The size (and shape) of the opening defined by this second x-ray passage area depends on the shape and size of the blocking member and its position relative to the primary x-ray passage area.
In one embodiment, movement of the blocking member is accomplished via a retracting arm that rotates a lever about a pivot point, thus causing the blocking member to move to an extended position. In this extended position, at least a portion of the blocking member physically obstructs a portion of the first x-ray passing region, thereby defining the second x-ray passing region. At least a portion of the blocking member is formed of a radiation attenuating material, thereby effectively reducing the pattern size of the x-ray signal that passes through the first x-ray passing region defined in the collimator plate. Thus, the second x-ray passing region has dimensions that are smaller than those of the first x-ray passing region.
These and other features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.